Senator Chuck Grassley (R-IA) speaks at an event for Republican presidential candidate Senator Marco Rubio (R-FL) at the Ramada Hotel in Urbandale, IA, Jan. 30, 2016. Reuters/Aaron Bernstein Sen. Chuck Grassley's (R-IA) approval rating among Iowa voters has taken a hit since Senate Republicans said they would refuse to consider President Obama's Supreme Court nominee, according to a new poll commissioned by liberal groups and released to Politico on Monday.

Just 42 percent of voters had a positive view of the historically popular Iowa senator, according to the poll, which was commissioned by the Constitutional Responsibility Project, a group pushing for the Senate to consider Merrick Garland's nomination, and the League of Conservation Voters. The polling was conducted by Hart Research Associates.

Grassley's approval is still relatively high, but in a Hart Research Associates poll from two years ago, 60 percent of voters had a positive reaction to Grassley. In a Des Moines Register/Mediacom poll conducted shortly after the late Supreme Court Justice Antonin Scalia's death in February showed Grassley earning 57 percent of Iowa voters' approval.

The six-term senator has long been popular in his state, but he may be more vulnerable in 2016 due to the Senate Republican blockade of Obama's Supreme Court nominee and the potential for Donald Trump to become the party's presidential nominee. As the chairman of the Senate Judiciary Committee, the committee in charge of hearings for judicial nominees, Grassley has become the unlikely focal point for Republicans' bid to stop Obama from appointing a Supreme court justice. Democrats have recruited former Lt. Gov. Patty Judge to challenge Grassley for his Senate seat.

According to the new poll, 43 percent of Iowa voters have a less favorable view of Grassley due to news coverage of the Supreme Court blockade, while 23 percent have a more favorable view of the senator.

Hart Research Associates surveyed 501 Iowa voters via phone from April 22-24 with a margin of error 4.5 percentage points.